Wintsche, Gergely2013-12-182013-12-182008http://hdl.handle.net/2003/3174210.17877/DE290R-11747We solve a bit more then the half of the XV.th. proposition of Leonardo Pisano who was also named as Fibonacci. We do it using only elementary tools of number theory. Our problem is explicitly what square number and the sum of a p prime of the form 4k+1 give a square number again.enGesellschaft für Didaktik der Mathematik510conference contribution